Some mechanism is necessary to contain the gas within a gas laser, while permitting free passage of the laser radiation. Endbell assemblies have been utilized to achieve this purpose. Endbell assemblies frequently are characterized by a hollow metal structure, physically attached to the laser body at one end and at the opposing end hermetically sealed with a window through which the laser light passes. The entire assembly is required to be vacuum-tight. These assemblies can be attached at opposing ends of the laser body. Although a window is the most common optical element with which to terminate a laser tube, other optical components including but not limited to prisms, birefringent filters, lenses, gratings, polarization devices, optical fibers, non-linear materials, mirrors and the like, can also be used.
The choice of window material is dictated first by its transparency to the desired laser light wavelengths, and then by practical considerations of ease of fabrication, cost, relative performance with use, etc. For ion lasers, a frequent material choice is crystalline quartz. In that case, the endbell consists of a crystalline quartz window portion and a metal endbell portion.
Endbell assemblies from a practical standpoint are not merely a metal portion which terminates in a window. For example, in an electrically excited laser, high voltages can exist on parts of the laser body other than the electrodes, in particular, metal portions of the endbells. These portions, if unprotected, may present a shock hazard to a laser operator. Because of this concern, an insulating, crystalline endbell component is interposed between the metal and window portions of the assembly, to separate and electrically isolate the window from the metal portion. This electrical isolation is highly desirable.
Other means exist for minimizing the chance of electrical contact by the user. In one, a metallic joint is formed between the metal portion of the endbell and the window itself. The structure of the assembly is such that the window portion is of larger diameter than the metal portion, and the latter is recessed from any possibility of exposure. Additionally, in another embodiment, an epoxy or non-conducting material could be applied to all metallic surfaces to eliminate metallic exposure and minimize the chance of an operator accident.
A variety of endbell designs and sealing techniques are possible. Essentially, a hard, brittle, anisotropic, high-expansion crystalline quartz window must be attached, by the endbell, to a metal tube on the laser body which frequently has dissimilar thermal expansion characteristics. This assembly must be clean, vacuum-tight, non-outgassing, durable, and bakable to about 400.degree. C. It must also be capable of removal and re-attachment several times and possess at least one rotational degree of freedom. Vacuum seals compatible with these requirements are known as "hard seals".
Hard seals have been accomplished in basically three ways, with powdered glass frits, brazes or deformable metal gaskets. Metal gaskets require a clamp to compress the gasket which results in a large and cumbersome seal assembly.
Frequently the window is sealed to the quartz endbell component by means of frit glass. The practice of employing frit materials is somewhat undesirable in that the joint is mechanically weak, and the processes required can frequently introduce contaminants which may adhere to the window surface itself, degrading its optical performance.
Accordingly, it would be an advancement to provide an endbell assembly where the window, insulating crystalline endbell portion, and the metal endbell portion could be sealed together without employing frit glass.